What do European or Chinese cycle commuters do when it rains?  Do they all just cram onto trolleys?
I've see photos of them wearing ponchos while riding in the rain. Did that myself a few times when I didn't have a car.
I commuted for five years by bike, rain or shine. All you need is good rain gear.
You find people who get put off by rain are not very commited to biking in the first place, and as a result haven't brought waterproof gear.

I bike->train->bike to work each morning insted of buying a car are doing a 42 mile round trip. When i have gotten lifts from work mates ive always felt like i have not properly woken up.

I agree with Beige about peak oil being serious when people have to bike in to work be good fun to have more people joining me in the morning :) assuming they still have a job :(

I've never minded getting rained on, what bothers me is that wet brakes can get you killed in a hurry.  So I'll bike home in the rain, but if it is raining when I wake up, I'll walk or take the car.
I've seen a very noticeable increase in people asking me about my bicycle and my commute, but no signs of any more people actually joining me in cycling.  Now that the cold, dark season is about to begin, I'm not expecting much.

Rain on a hot summer day can be nice.  On a cold, dark, fall night, not as much fun, but, yes, certainly doable.  Chance of light rain or even actual drizzle don't deter me, but I'm not motivated enough yet to set out into a downpour with continued rain and storms forecast.  And yet people here think I'm some sort of cycling nut.  (I see studded tires in the bike shop, so there is plenty of room for me to became a real nut someday.)

I do combine the rainy day drives with running the errands that are far easier to do with the car.  Most of the places I want to go to are much closer to work than home, most of them in between work and home, so it's a reasonable trip combination.